An emergency responder and volunteers, including Carlos Arredondo in the cowboy hat, push Jeff Bauman in a wheel chair after he was injured in an explosion near the finish line of the Boston Marathon Monday, April 15, 2013 in Boston. At least three people were killed, including an 8-year-old boy, and more than 170 were wounded when two bombs blew up seconds apart. (AP Photo/Charles Krupa) Purchase photo reprints »

An emergency responder and volunteers, including Carlos Arredondo in the cowboy hat, push Jeff Bauman in a wheel chair after he was injured in an explosion near the finish line of the Boston Marathon Monday, April 15, 2013 in Boston. At least three people were killed, including an 8-year-old boy, and more than 170 were wounded when two bombs blew up seconds apart. (AP Photo/Charles Krupa) Purchase photo reprints »

BOSTON — Doctors on Tuesday described objects “clearly designed to be projectiles” removed from patients injured in Monday’s Boston Marathon explosions.

“There’s no question some of these objects were implanted in the device for the purpose of being exploded when the device went off,” said Dr. Ron Walls, Brigham & Women’s chair of emergency medicine.

They are “ball-bearing type” objects, “just a little larger than a BB,” said Walls, describing metallic beads about two to three millimeters in diameter. Surgeons also removed more than a dozen small “carpenter-type nails” about a centimeter to an inch in length from one patient, he said.

Citing a person briefed on the probe into the explosions, the Associated Press reported Tuesday afternoon that the bombs were made out of pressure cookers loaded with metal and ball bearings.

Authorities have told doctors to preserve as evidence any foreign material they remove from patients so it can be used as evidence.

At Boston Medical Center, five patients had their limbs amputated and others endured abdominal surgery after being wounded by what Dr. Andrew Ulrich, executive vice chairman of BMC’s Department of Emergency Medicine, called “scrap metal like material.”

All told, Boston Medical Center took in 23 blast-related patients. Tuesday, BMC still had 19 of those patients in their care. Ten of them are in critical condition, and three others are in serious condition, said BMC trauma surgeon Dr. Tracey Dechert.

“The vast majority were low extremity patients, some of them had what we call traumatic amputations, due obviously to the bomb,” Dechert said. “Others had mangled extremities to the point that those patients obviously required operations, and still will require operations.”

Ulrich described three hours of controlled chaos at Boston Medical Center Monday, as ambulances roared in with maimed marathon spectators and frantic doctors and nurses raced in behind them to help.

“Our patients range from 5 to 78 years old,” said Ulrich. The 5-year-old is in critical condition.

“The injuries we saw were very much blast-related injuries, where we saw a lot of extremity injuries, a lot of limb amputations a lot of wounds that were from the blast itself,” he said.

“We also saw some patients with head injury as well as abdominal injury,” Dechert added.

“Many will need more operations in the coming days,” Dechert said, “and we expect that those in critical condition will do well in time.”

Ulrich said most of the explosion patients were not runners. “Most of them were in the crowd,” he said.

He said authorities were quickly at the hospital, gathering evidence.

He also said 90 percent of the BMC staffers showed up to work during that time. Some, Ulrich said, came right off the race course and into the ER.

“I’ve never been prouder to work here,” he said. “The staff response was outstanding.”

Between Brigham & Women’s Hospital and its Faulkner Hospital location, 44 patients were admitted, 13 of whom are in the operating room, and six of whom are in critical condition. The youngest patient was 16 and the oldest was 62. Patients are “about half and half” men and women.

At the Brigham, nine went to the operating room, five in critical condition. One had a below-the-knee amputation. Two other patients have “threatened limbs.” The patients are being watched very closely by trauma surgeons and nurses. “These limbs are still in jeopardy,” he said.

Of the nine that went into surgery, two returned to the operating room Tuesday. Several more will be returning to the operating room.

At Faulkner Hospital, there were 13 patients, four of whom required surgery, with no amputations.